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Abstract:

A coating device for coating a medical device with a drug-eluting
material uses an in-process drying station between coats to improve a
drug release profile. The drying station includes a dryer having a
telescoping plenum which provides a drying chamber for the stent or
scaffold to reside while a heated gas is passed over the stent/scaffold.
The drying chamber improves efficiency in drying, predictability or drug
release rate, uniformity of coating material properties lengthwise over
the stent/scaffold and provides a platform that can effectively support
stents that are over 40 mm in length.

Claims:

1. A method for applying a composition to a stent, comprising the steps
of: spraying the composition on the stent; and drying the stent,
including the steps of moving a drying chamber towards the stent to place
the stent within the drying chamber, applying a drying gas to dry the
stent, and after drying the stent, moving the drying chamber away from
the stent.

2. The method of claim 1, further including the steps of displacing the
stent towards a dryer and only when the stent is over or under a mouth of
the dryer, moving the drying chamber towards the stent.

3. The method of claim 1, wherein the moving step includes the drying
chamber being raised or lowered when a mandrel supporting the stent is
over or under, respectively, the drying chamber so that the stent passes
through an opening in the drying chamber and the mandrel rests on the
drying chamber.

4. The method of claim 3, further including the step of disposing a
proximal end and the distal end of the mandrel within alignment grooves
of the drying chamber, and rotating the mandrel when the drying gas is
applied and the ends sit in the grooves.

5. The method of claim 1, wherein the stent is supported on a mandrel,
the drying step further including an actuator gripping a distal end of
the mandrel when the stent is over or under the drying chamber.

6. The method of claim 1, wherein the moving step includes the step of
expanding a plenum of a dryer to position a mouth of the dryer near the
stent.

7. The method of claim 6, wherein the expanding a plenum of the dryer
further includes the step of displacing a first housing of the dryer
using an actuator mechanism coupled to the first housing.

8. The method of claim 7, the dryer including a second housing coupled to
a gas supply, the first and second housing forming at least a portion of
the plenum, further including the step of sliding the first housing
relative to the second housing when the plenum is expanded.

9. A dryer for drying a stent, comprising a first housing configured for
being connected to a gas supply; a second housing movable within the
first housing, the second housing including a drying chamber in fluid
communication with a mouth of the dryer and configured to receive and
support a mandrel, the mouth being located near a base of the drying
chamber, and a diffusion chamber disposed below the mouth.

10. The dryer of claim 9, further including a first actuator coupled to
the second housing and configured to displace the second housing relative
to the first housing.

11. The dryer of claim 9, further including a second actuator capable of
supporting an end of the mandrel and aligning it with the mouth.

12. The dryer of claim 11, wherein the drying chamber has a first opening
having a width and length greater than a width and length of a stent and
the mouth size is about the same size as the opening.

13. The dryer of claim 9, further including a cap coupled to the first
housing, wherein a plenum of the dryer is formed by the first housing,
second housing and cap.

14. The dryer of claim 9, wherein the dryer is configured such that the
plenum has a first size when a stent is disposed within the drying
chamber, and a second size when the drying chamber is devoid of a stent,
the first size being greater than the second size.

15. The dryer of claim 9, wherein the diffusion chamber includes a spacer
and a screen that is received within the second housing.

16. The dryer of claim 9, wherein a plurality of diffusion chambers are
disposed within the diffuser housing.

17. The dryer of claim 9, wherein the dryer is a telescoping dryer.

18. The dryer of claim 9, wherein the second housing further includes a
first and second groove formed at opposite ends of the drying chamber for
retaining a stent-supporting mandrel during drying.

19. A stent coating system, comprising: a sprayer; a telescoping dryer; a
linear actuator for moving a stent-supporting mandrel between the
telescoping dryer and the sprayer; and a rotary actuator for rotating the
stent-supporting mandrel during drying and spraying.

20. The stent coating system of claim 19, wherein the telescoping dryer
has a variable length plenum configured to have a first length when a
stent is being sprayed and a second length when the stent is being dried,
the second length being greater than the first length.

21. The stent coating system of claim 19, wherein the telescoping dryer
includes an actuator mechanism for displacing a drying chamber of the
telescoping dryer towards and away from the stent when the stent is
located over a mouth of the telescoping dryer.

22. The stent coating system of claim 19, further including a controller
for controlling a gas supply temperature to the telescoping dryer, the
controller configured for providing a steady state gas supply and
switching between an idle state and an in-use state when the stent is
being sprayed and dried, respectively.

23. The stent coating system of claim 19, wherein the telescoping dryer
includes means for both aligning the stent within a drying chamber and
stabilizing the stent during drying.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to drug-eluting medical devices; more
particularly, this invention relates to processes for controlling the
interaction among polymer, drug and solvent, and the release rate of a
drug for drug eluting medical devices.

[0003] 2. Background of the Invention

[0004] Strict pharmacological and good mechanical integrity of a drug
eluting medical device are required to assure a controlled drug release.
Significant technical challenges exist when developing an effective and
versatile coating for a drug eluting medical device, such as a stent.

[0005] A coating may be applied by a spray coating process. A drug-polymer
composition dissolved in a solvent is applied to the surface of a medical
device using this method. The amount of drug-polymer to be applied has
been expressed as a target coating weight, which corresponds to the
weight of the coating after a substantial amount of the solvent is
removed.

[0006] Previous efforts to produce a more consistent and stable drug
release profile have been met with challenges. Prior efforts have focused
on the type or structure of the polymer carrier for a drug, and the type
of solvent used. However, these improvements have not been able to
satisfactorily meet the needs for certain clinical applications, or
provide a morphology that can be widely used.

[0007] A "drug release profile", or "release profile" means the
morphology, or characteristics of a drug-eluting matrix that delivers an
expected therapeutic behavior after being placed within a body. A drug
release profile, or release profile therefore informs one of such things
as the predictability of the release rate, variation, if any, in the
release rate over time or on a per unit area basis across a drug-eluting
surface.

[0008] It has been previously discovered that a significant improvement in
the ability to tailor a drug release profile to suit a particular
objective such as producing a specific release rate, uniformity in the
release rate over a drug eluting surface, and/or uniformity in a
production setting (high throughput) lay in obtaining more precise
control over the amount of solvent present, or rate of solvent removal.
The criticality of solvent removal, distribution, etc. generally depends
on the drug-polymer-solvent formulation and particular objectives. While
it was already known that the morphology of a drug-polymer matrix is
influenced by the presence of a solvent, it was later discovered that
this interaction played a more significant role than previously thought.
Based on this conclusion, a more effective process for controlling the
amount of solvent-polymer-drug interaction was sought. It was found that
the coating weight per spray cycle and manner in which solvent was
removed, in connection with the coating thickness was an important
consideration.

[0009] A relatively high coating weight per spray cycle has been sought in
the past, because this minimizes process time and increases throughput.
Maintaining control over the amount or rate of solvent removal is,
however, challenging unless an applied coating layer is relatively thin.
If the applied layer is too thick the removal of the solvent becomes more
difficult to control or predict. When the solvent is removed from a thick
layer, therefore, the potential for undesired interaction among the
solvent, polymer and drug, and related problems begin to impair the
ability to retain control over the release profile.

[0010] Process conditions can affect the desired morphology. For example,
if there is excess residual solvent, i.e., solvent not removed between or
after a spray cycle, the solvent can induce a plasticizing effect, which
can significantly alter the release rate. Therefore, it can be critically
important to have a process that produces a coating with consistent
properties--crystallinity, % solvent residue, % moisture content, etc. If
one or more of these parameters are not properly controlled, such that it
varies over the thickness or across a surface of a drug-eluting device,
then the release profile is affected. One or more of these considerations
can be more critical for some drug-polymer-solvent formulations than for
other formulations.

[0011] To facilitate the incorporation of a drug on a stent, spraying a
low solid percent polymer/drug solution over the stent followed by
removing the solvent has become feasible in controlling the amount of
drug (in micrograms range) deposited on the stent and the release
profile. A good coating quality benefits from using this spray technique,
i.e., properties such as the crystallinity, % solvent residue, and %
moisture content are more controllable as the coating weight is built up
over several applied coatings.

[0012] Previous studies of the drying effect on drug release indicated a
need for an optimal in-process or inter-pass drying technique to remove a
solvent on the coated stent after each spray cycle. This is a critical
step in producing more stable products while retaining a high throughput.

[0013] The properties of a solvent, e.g., surface tension, vapor pressure
or boiling point, viscosity, and dielectric constant, used in dissolving
a polymer have a dominant effect on the coating quality, coating process
throughput, drug stability, and the equipment required to process it. A
solvent can, of course, be removed by applying a heated gas over the
stent. However, this drying step must be carefully controlled in order to
achieve the desired end result. A uniform and efficient heat transfer
from the gas to the coating surface must also take place.

[0014] The evaporation rate of a suitable solvent has an inverse
relationship with the coating thickness (generally inversely proportional
to the thickness) for a thin film coating. And the resistance increases
non-linearly as the coating thickness increases. As alluded to earlier,
this non-linearity should be avoided. When the coating thickness is not
too high more uniformity and control can be achieved in removing the
solvent. As a result, a more consistent drug release profile is obtained
because there is the least drug-solvent-polymer interaction, solvent
plasticizing and drug extraction rate. It is therefore desired to achieve
more control over, not only the uniformity of properties across the
coating thickness and along the length of the stent, but also the ability
to remove solvent. This is because residual solvent on the drug eluting
stent may induce adverse biological responses, compromise coating
properties, induce drug degradation, and alter release profile.

[0015] Thus, it has been determined that a release rate can be better
controlled by applying many coats of a low percentage solution, e.g., 5%
of the final coating weight, with a drying step between each spray cycle.
Thus, in this example 20 coats are needed to produce the target coating
weight. In order to make this coating process more feasible as a
production-level method, while maintaining control over the solvent and
solvent-drug-polymer interaction, as just discussed, an efficient
in-process drying step is needed.

[0016] Effective ways to remove residual solvent in the applied coating
becomes more important for coating formulations that are more sensitive
to a residual solvent level. As explained above, excessive remaining
solvent impacts the coating morphology and property. For example, in the
case of a coating formulation used for a polymer scaffold, e.g., PLLA,
residual solvent left in the coating can induce phase separation between
the drug and polymer because the drug and polymer are not miscible. This
can cause variation of the drug release rate and adversely impact the
physical properties of the coating. It is therefore desirable to achieve
an optimized in-process dry nozzle design to ensure the removal of most
of the residual solvent between successive spray cycles. Examples of
dryers seeking to achieve this objective are described in US20110059228
and US20110000427.

[0017] For example, US20110000427 proposes using an external heat nozzle
design having a narrow opening producing a drying gas exiting from the
dryer plenum at relatively high velocity. This arrangement requires
precise alignment between the stent and heat nozzle for uniform drying.
The design can introduce extensive and interfering mixing of outside air
into the gas stream before contacting the stent or scaffold; this mixing
of outside air is uncontrolled and causes variation in the temperature
across the drying area. Additionally, the high velocity gas causes the
stent to oscillate, which can be problematic for longer-length stents,
such as those intended for peripheral vessels.

[0018] There is a continuing need for obtaining a better control over the
drug-eluting product. Specifically, there is a need to develop an
inter-pass drying process that is better able to remove solvent to
achieve improved rate of release of a drug, uniformity of release rate
over the stent length and/or the effectiveness of a drug when released
from the coating. It is also desirable to reduce processing time when
applying a drug-eluting coating.

SUMMARY OF THE INVENTION

[0019] The invention proposes an in-process dryer for maximizing
in-process drying efficiency and uniformity for improving the product
quality (e.g. coating and its drug release consistency). A dryer and
associated process according to the invention can also obviate the need
for an oven step which has been relied on to remove residual solvent,
thereby streamlining the manufacturing process.

[0020] A dryer nozzle according to the invention has a wider mouth or exit
from the plenum than previously proposed stent dryer designs. With this
design mean gas velocity at the dryer nozzle is reduced over earlier
dryer designs, so that there is less or no influence by the surrounding
ambient air and less oscillations of the stent during drying. In a
preferred embodiment the dryer is constructed as a telescoping dryer
assembly, although other designs are contemplated, e.g., a dryer nozzle
that is moved into and out of position as a single unit connected to a
flexible gas supply. A shield surrounds the drying region, or drying
chamber to isolate heated gas from surrounding cooler ambient air. The
stent (or scaffold) is disposed within this drying chamber during the
drying step. The dryer nozzle is retractable, which allows clearance for
movement of the sent or scaffold between spraying and drying stations.
The feature of a retractable dryer nozzle also simplifies drying
operations, such as concerns aligning the stent with the mouth or exit.

[0021] A dryer according to the invention addresses alignment issues and
uneven drying seen in prior designs by ensuring full coverage and uniform
heat application. In addition, the influence of ambient air in the drying
operation is effectively minimized or eliminated. Tests have shown that
the temperature within the shielded area of the drying chamber and just
above it is at a constant temperature, indicating that no ambient air is
drawn into the drying chamber. Since the hot air within the drying
chamber is at a slightly higher pressure than the surrounding ambient
air, ambient air is prevented from being drawn into the drying chamber.
The dryer nozzle includes internal diffusers, e.g., stacked spacer and
screen assemblies, to uniformly mix the heated drying gas, resulting in a
temperature uniformity of within 1 degree C. across the stent drying
area.

[0022] Accordingly, an inter-pass dryer, according to the invention, that
is used in a stent coating process improves on the art by providing an
apparatus and method for forming a drug-eluting coating that offers
greater control over the release rate for a drug and less undesired
interaction between residual solvent and the drug-polymer matrix in the
coating. The term "inter-pass drying" means drying, or removing solvent
between one, two, three or more spray passes. The weight of material per
coat is in some embodiments are very light, about 5% of the total coating
weight according to one embodiment. This means, for this particular
embodiment, 20 coats are needed to reach 100% of the coating weight.

[0023] In view of the foregoing, the invention provides one or more of the
following additional improvements over the art.

[0024] According to one aspect of invention, a method for applying a
composition to a stent, comprising the steps of spraying the composition
on the stent; and drying the stent, including the steps of moving a
drying chamber over the stent, applying a drying gas to dry the stent,
and after drying the stent, moving the drying chamber away from the
stent.

[0025] According to another aspect of invention, a dryer nozzle for drying
a stent includes a first housing configured for being connected to a gas
supply; a second housing movable within the first housing, the second
housing including a drying chamber in fluid communication with a mouth of
the dryer nozzle and configured to receive and support a mandrel, the
mouth being located at a base of the drying chamber, and a diffusion
chamber disposed below the mouth.

[0026] According to another aspect of invention, a stent coating system
includes a sprayer; a telescoping dryer nozzle; and a linear actuator for
moving a stent-supporting mandrel between the telescoping dryer nozzle
and the sprayer. The system may further include a rotary actuator for
rotating the stent-supporting mandrel to improve consistency and
uniformity of solvent removal.

INCORPORATION BY REFERENCE

[0027] All publications and patent applications mentioned in the present
specification are herein incorporated by reference to the same extent as
if each individual publication or patent application was specifically and
individually indicated to be incorporated by reference. To the extent
there are any inconsistent usages of words and/or phrases between an
incorporated publication or patent and the present specification, these
words and/or phrases will have a meaning that is consistent with the
manner in which they are used in the present specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1A is side view of a dryer assembly in a first, retracted
position according to one aspect of the disclosure.

[0029]FIG. 1B is side view of the dryer assembly in a second, expanded
position according to another aspect of the disclosure.

[0030]FIG. 2 summarizes a process for coating a stent including a
spraying step and in-process drying step using the dryer assembly of FIG.
1.

[0032]FIG. 4 is a front perspective, exploded assembly view of the dryer
assembly showing component parts according to a preferred embodiment.

[0033]FIG. 5 is a perspective view of a base cap of the dryer assembly of
FIG. 4.

[0034]FIG. 6 is a perspective view of a diffuser housing of the dryer
assembly of FIG. 4.

[0035] FIGS. 7A and 7B are perspective views of left and right grippers of
a mandrel gripper of the dryer assembly of FIG. 4.

[0036]FIG. 8 is a perspective view of a base housing of the dryer
assembly of FIG. 4.

[0037]FIG. 9 is a schematic of a control system that may be used with the
dryer assembly to minimize transient flow or wait time and conserve dryer
resources while a coating is being applied to a stent.

DETAILED DESCRIPTION OF EMBODIMENTS

[0038] According to a preferred implementation of the invention, a sprayer
and dryer nozzle is used to form a drug-eluting coat on a surface of a
stent. A stent is an intravascular prosthesis that is delivered and
implanted within a patient's vasculature or other bodily cavities and
lumens by a balloon catheter for balloon expandable stents and by a
catheter with an outer stent restraining sheath for self expanding
stents. The structure of a stent is typically composed of scaffolding,
substrate, or base material that includes a pattern or network of
interconnecting structural elements often referred to in the art as
struts or bar arms. A stent typically has a plurality of cylindrical
elements having a radial stiffness and struts connecting the cylindrical
elements. Lengthwise the stent is supported mostly by only the flexural
rigidity of slender-beam-like linking elements, which give the stent
longitudinal flexibility. Examples of the structure and surface topology
of medical devices such as a stent and catheter are disclosed by U.S.
Pat. Nos. 4,733,665, 4,800,882, 4,886,062, 5,514,154, 5,569,295, and
5,507,768.

[0039] As discussed earlier, one aspect of the stent coating process that
has been simplified, or improved, as a result of the dryer according to
the disclosure, is the ability to predict more consistently the rate of
solvent removal and variation of that rate over the length of the stent.
Increasing the predictability of a solvent's presence in the applied
coating, or remaining when determining a final weight can greatly
increase the ability and/or efficiency in which a predictable release
rate for a drug can be provided in a medical device, in the form of an
applied coating.

[0040] Moreover, as the design or desired loading of polymer-drug on the
stent is determined from the measured weight, it will be readily
appreciated that there needs to be an accurate, reliable and repeatable
process for being able to determine the amount and distribution of
solvent remaining over the length of the stent. This is especially true
when less volatile solvents are used, e.g., DMAc as opposed to the more
volatile solvent Acetone. Since it is expected that a greater percentage
of solvent will remain after drying for solvents having higher boiling
points, the coating is more susceptible to variations in a solvent's
presence over the stent surface and/or across the coating thickness. Also
when drying a polymer Acetone mixture, the rate and uniformity of drying
affects the % crystallinity and thus the amount of locked in residual
solvent.

[0041] The disclosure provides examples of spraying/drying components
suited for addressing the previously discussed drawbacks and limitations
in the art pertaining to a drug-eluting coating applied via a
drug-polymer dissolved in a solvent.

[0042] FIGS. 1A-1B show side views of a telescoping dryer 10 (dryer 10)
according to one aspect of the disclosure. FIG. 2 shows a flow process
for applying, via a spray apparatus, a composition, i.e., drug-polymer
coating dissolved in a solvent, to a stent including applying one or more
coats of the sprayed composition followed by a drying step that may
include using dryer 10. Accordingly, the dryer 10 may be included as a
component to a stent coating apparatus. Such a stent coating apparatus
implementing the process of FIG. 2 includes a sprayer, the dryer 10 and
actuators for placing the stent between a spraying area or chamber and a
drying area for performing a drying step, or solvent removal step,
between each of several coatings of composition sprayed onto the stent.
Examples of a stent coating apparatus that may adopt principles of the
disclosure are described in U.S. patent application Ser. Nos. 12/497,133;
12/027,947 and 11/764,006. In these examples, the dryer(s) described
therein may instead utilize a dryer according to the disclosure, as will
be understood.

[0043] Referring, briefly, to side views of the dryer 10 as depicted in
FIGS. 1A-1B, after one or more coatings are applied by a sprayer, the
stent (supported on a mandrel 15) is moved into position over the dryer
10, as indicated in FIG. 1A. Mandrel grippers 60 then engage a distal end
15a of the mandrel 15 to account for any slight misalignments of the
stent position over the dryer exit or mouth and stabilize the stent as it
rotates and is impacted by gas exiting from the dryer plenum. A diffuser
housing 30 telescopes or deploys from a base housing 20 (using a linear
actuator mechanism 50) to place or enclose the stent within a drying
chamber 32, as indicated in FIG. 1B. After the drying step is complete,
the diffuser housing 30 retracts back into the base housing 20, the
grippers 60 are released from the mandrel end 15a and the stent moved
back to the spraying station to apply the next coating. These steps of a
stent coating process are summarized in FIG. 2.

[0044] FIGS. 1A and 1B show the stent positioned above the dryer 10.
However, the stent may alternatively be located below the dryer 10. In
such an arrangement, the drying chamber 32 would be placed above the
stent and the drying gas directed downward, rather than placed below the
stent and directed upward, respectively, as depicted in these drawings.

[0045] The stent, supported on the mandrel 15, is rotated by a rotary
mechanism (not shown) coupled to the mandrel 15 as the sprayer applies a
drug-polymer dissolved in a solvent, e.g., DMAc or Acetone, to the
surface of the stent. This rotary mechanism is also used to rotate the
stent while it is disposed within the drying chamber 32 to facilitate
uniform removal of solvent about the circumference of the stent during
drying. A mass of heated gas exits from the mouth of the dryer (at a base
of the drying chamber 32) to accelerate the evaporation, or boiling-off
of solvent from the coated stent surface. In a preferred embodiment, this
sprayer-dryer coating process is repeated until a final coating weight of
drug-polymer and remaining solvent is measured. During each drying stage
the gas is capable of producing a uniform heat transfer across the
surface of stents or scaffolds, even for stents or scaffolds having
lengths of 100 mm, 150 mm, and 200 mm.

[0046] A coating process according to FIG. 2 may be preprogrammed, or
programmed on the fly to adjust parameters such as number of coats, or
passes with the sprayer between drying steps, number of cycles of
spraying and drying, etc. These and related parameters may be governed by
the polymer-drug or solvent used, type of stent or medical device being
coated, e.g., surface geometry. In particular embodiments the protocol
for coating a stent may be governed by a predetermined number of coating
cycles, i.e., spraying then drying, based on an analytically determined
final coating weight, or by intermittent weighing of the stent to
determine the number of cycles needed to arrive at the target coating
weight.

[0047] FIGS. 3 and 4 show an assembled rear perspective view and exploded
front perspective assembly view, respectively, of the dryer 10. A mouth
or exit of the dryer 10 is present at the base of the drying chamber 32
and has dimensions the same as the opening to the drying chamber 32; in
other words, the walls forming the drying chamber 32 are parallel to each
other or the cross-sectional area of the entrance to the drying chamber
32 is the same as the cross-sectional area of the opening through which
the stent passes when entering/exiting the drying chamber 32. A gas
supply is connected to an entrance of the dryer 10 provided by the base
housing 20. The drying gas, e.g., heated nitrogen or air, is supplied
through a gas supply 2b connected to a heater assembly 2. The heater
assembly 2 includes a tubular conduit with heating coils exposed to the
gas stream as it travels towards the dryer entrance 9. The coils are
connected to a power source via a power connection.

[0048] A plenum of the dryer 10 is formed by internal volumes of the base
housing 20, the diffuser housing 30 and a base cap 70. Perspective views
of the base cap 70 and diffuser housing 30 are illustrated in FIGS. 5 and
6, respectively. A hole in the dryer base housing 20 (hidden from view)
is formed to co-align with a similar shaped hole in the base cap 70 (also
hidden from view) to provide a passage for gas into the interior of the
base cap 70. The hole or passage for gas through the base housing 20
includes a threading to sealingly engage a complimentary threaded fitting
2c of the heated gas supply. Gas entering through this passage passes
directly into the interior of the base cap 70, exits through a hole 72
formed at the top of the base cap 70 then passes up through the diffuser
housing 30. The base cap 70 and diffuser housing 30 are contained within
the base housing 20 when fully assembled.

[0049] To account for any thermal energy loss for gas near the walls of
the housings 20, 30 one or more mixing regions are provided within the
diffuser housing 30 so that the gas entering the drying chamber 32 has a
more uniform heat transfer across the length of the stent. Preferably
three mixing regions are used for dryer 10. Each mixing region is formed
by a diffuser screen 42 and spacer 40. Each screen and spacer are stacked
on top of each other, as indicated in FIG. 4. From tests it was found
that three spacers and screen assemblies were sufficient to cause no more
than about a 1 degree Celsius temperature difference within the drying
chamber 32 during a drying step.

[0050]FIG. 4 indicates the order of assembly of the portions forming the
plenum of the dryer 10, i.e., diffuser housing 30, base cap 70, base
housing 20 and spacers and screens 40, 42. The three spacers and screens
40, 42 are placed inserted within the diffuser housing 30 and may be held
in place by pins at the edge 31. The diffuser housing 30 is placed within
the dryer base 20 through a bottom edge 24 thereof. The dryer housing 20
and diffuser housing 30 are then placed on the base cap 70 such that a
lower edge 24 of the dryer housing 20 rests on a lower flange 76 of the
base cap 70. The lower spacer 40a rests on an upper surface 74 of the
base cap 70. The base housing 20 is press-fit onto the base cap 70 to
provide a fluid-tight seal between the walls of the two structures. This
assembled configuration of the dryer 10 is depicted in FIG. 1A.

[0051] As mentioned above, gas travels from the gas supply into the
interior of the base cap 70, though the exit hole 72 and then through the
diffuser housing 30. When the diffuser housing 30 is lifted up to
position the stent within the drying chamber 32 (FIG. 1B), the spacer 40a
lifts off the surface 74 of the base cap 70. To ensure gas passes
directly from the base cap into the diffuser housing 30, a tight but
slidable fit is formed between the interior walls of the housing 20 and a
lower flange 31 of the diffuser housing 30. In essence, this fit
maintains a desired gas pressure within the plenum while the dryer 10 is
expanded (or housing 30 lifted) to receive the stent in the drying
chamber 32, and while allowing the diffuser housing 30 to be moved up and
down by the actuator 50 while the housing 20 and base cap 70 remain
stationary (FIG. 1B). The travel upwards of the diffuser housing 30
within the base housing 20 is limited by the flange 31. After the
diffuser housing 30 has traveled a sufficient distance (to place the
stent within the drying chamber 32) the flange 31 abuts an upper surface
of the opening 22 of the diffuser housing 20, thereby preventing further
upward movement. To promote the seal between the interior walls of the
housings 20, 30, therefore, the edge 31 slides against along the walls of
the housing 20 as the diffuser housing 30 is being moved upwards and
downwards within the housing 20 by the actuator 50. More generally, the
sliding fit between these telescoping parts enables a plenum pressure to
be achieved and maintained (no leaks) while the dryer 10 is
retracted/shortened and expanded/lengthened.

[0052] As just alluded to, the aforementioned structure, i.e., housings
20, 30 and base cap 70, and mechanism 50 that form the plenum for the
dryer 10 may be thought of as a telescoping dryer. Prior to the stent
being positioned over the drying chamber 32, the diffuser housing 30 is
retracted within the base housing 20 to provide clearance for the stent
and mandrel 15 to be linear displaced from the spray station to a
position over the drying chamber 32. The dryer plenum is then essentially
elongated or expanded to bring the stent into the drying chamber 32 of
the diffuser housing 30. Thus, a "telescoping dryer assembly" is intended
to mean an arrangement of housings forming a plenum that slide inward and
outward in overlapping fashion in a manner analogous to how a hand
telescope slides inward and outward in an overlapping fashion, to thereby
provide a variable length channel or internal passage for a pressurized
fluid to pass through, i.e., a variable length plenum.

[0053] Referring to FIGS. 3 and 4, the dryer 10 components and actuating
mechanisms 55 and 50 are secured to a plate 14, which is connected to a
pair of blocks 16 and brackets 12. The actuating mechanism 55 is used to
displace left and right grippers 62, 64 towards and away from each other
to grip and release, respectively, the distal end 15a of the mandrel 15;
this movement being indicated by the left and right arrows G in FIG. 3. A
detailed view of each gripper 62, 64 is shown in FIGS. 7A-7B.

[0054] The actuating mechanism 50 (e.g., one or more hydraulic actuators,
such as air cylinders, operated as part of a servomechanism
pre-programmed or controlled by a computer processor to produce the
desired movement in the housing 30 in accordance with a drying/spraying
process as shown in FIG. 2) is used to raise and lower the diffuser
housing 30; this movement indicated by the up and down arrows L in FIG.
3. A connecting plate 54 has a rim, which is placed over the diffusing
housing and secured to a top ledge 34 of the diffuser housing 30, and a
flange 54a that is secured to a platform 54b that is movable up and down
by a pair of air cylinders 56a, 56b. Thus, the actuator causes the plate
54 to pull up on the housing 30 when the plenum is being extended or
lengthened (FIGS. 1B and 3), and push down on the housing 30 when the
plenum is being retracted or shortened (FIG. 1A). FIG. 3 shows the dryer
10 configuration with the housing 30 raised to position the stent within
the drying chamber 32 and the gripper pair 62, 64 gripping the end 15a of
the mandrel 15. This is also the configuration shown in FIG. 1B.

[0055]FIG. 5 shows a perspective view of the base cap 70, with the
portions identified as previously described. As can be appreciated by
comparing the contours of the base cap top surface 74 and the housing 30
(FIG. 6), the dryer 10 preferably has an elongate shape with rounded
ends, just as the drying chamber 32 is shaped to receive the stent or
scaffold. The base cap 70 may be formed to have walls that are thicker
than the housings 20, 30 (see FIG. 1A) to provide increased insulation
capability. Since the gas enters here and is redirected 90 degrees to
exit from hole 72, there is a greater heat loss possibility than after
the gas exits through hole 72. As such, the walls are made thicker and
preferably they are made from PEEK. As described earlier, a last step of
the assembly for dryer 10 is to press fit the housing 20 (with diffuser
housing 30 inside) onto the base cap 70. This last step essentially seals
the dyer 10 and forms the interior space for the dryer plenum.

[0056]FIG. 6 shows a perspective view of the diffuser housing 30, with
features of this structure as previously described. The drying chamber 32
is elongate with rounded ends to receive the stent or scaffold therein.
The drying chamber 32 provides a surrounding shield or walls 30b that
rise up from the ledge 34, which ledge 34 locates the exit opening from
the plenum (the dryer mouth) into the drying chamber 32, thereby also
reflecting a depth of the drying chamber 32. Gas flowing near the stent
and within the drying chamber 32 may exit from the plenum at a relatively
low velocity which favorably limits the amount of regress or interference
from ambient air. As mentioned earlier, by providing a shield and gas at
a lower exit velocity which maintains its heat when exposed to the stent,
there is an alternative to the dryer assemblies described in
US20110059228 and US20110000427. The mouth of the dryer is located at the
base of the drying chamber. The opening provided for the stent is about
the same size as the mouth size (not shown in the drawings).

[0057] FIGS. 7A and 7B show perspective views of grippers 62, 64,
respectively. Each has arms 58a, 58b that form holes 57a, 57a at lower
ends thereof to secure the grippers 62, 64 to the actuator mechanism 55
(FIG. 4) using bolts. At the head of the grippers 62, 64 are semicircular
and complimentary slots 63a, 63b that are aligned to capture the distal
end 15a of the mandrel 15 within a circular passage formed when the slots
63a, 63b are brought together by the actuator mechanism 55 (e.g., one or
more hydraulic actuators, such as air cylinders, operated as part of a
servomechanism pre-programmed or controlled by a computer processor to
produce the desired movement in the grippers in accordance with a
drying/spraying process as shown in FIG. 2). V-shaped sections 66, 67,
aligned with slots 63a, 63b, function as guiding surfaces to urge the
mandrel 15 into the semicircular slots 63a, 63b (see FIGS. 1B and 3). As
can be appreciated by inspecting the spacing between the V-shaped section
66 and slot 63a of gripper 62, the closer spacing between the V-shaped
section 67 and slot 63b of the gripper 64, the dimension G1 in FIGS.
7A-7B, and the interlocking manner in which the grippers engage the
mandrel, as shown in FIG. 3, the V-shaped section 67 is disposed within
the space 69 of the gripper 62 when the mandrel end 15a is engaged by the
grippers 62, 64. When the stent is moved into position above the drying
chamber 32, the grippers 62, 64 come together. Any misalignment of the
mandrel end 15a is adjusted by the V-shaped sections engaging the mandrel
end 15a and urging it towards alignment with the slots 63a, 63b. When the
grippers 62, 64 are moved into contact with each other, the mandrel end
15a is held in place within the circular passage formed by the slots 63a,
63b. This ensures that the stent is being positioned properly within the
drying chamber 32 and held in position when the drying gas is passed over
the stent. The mandrel end 15a may rotate while it is disposed within the
circular passage formed by the slots 63a, 63b.

[0058] The shield 30b forming the drying chamber 32 includes a first notch
36 disposed at one rounded end, and a second notch 38 disposed at a
second or opposed rounded end. These notches 36, 38 are used to allow the
mandrel that the stent sits on to lower the stent to within the drying
chamber 32 during the drying. When the gas exits, even at a low velocity
the stent will oscillate since it rotates which presents a varying
surface area to the gas exiting (in addition to the non-laminar or
transient flow in and around the stent). The problem of oscillations is
especially noted for stents that are 40 mm and longer, e.g., stents (or
scaffolds) intended for the superficial femoral artery. To meet these
needs the dryer 10 includes a support for the mandrel 15 distal end 15 a,
i.e., mandrel grippers 60, in addition to the notches 36, 38. With the
additional support provided by grippers 60 the stent becomes effectively
fixed-supported at the mandrel distal end 15a when disposed over the
dryer mouth (exit of the plenum), yet is still capable of being rotated
about the mandrel axis by a rotary mechanism coupled to the mandrel. This
support may be achieved without interference with drying and prevents
contact between the stent/scaffold and the walls 30b or mandrel 15 as the
gas passes over the stent/scaffold.

[0059] The stent is mounted onto the mandrel 15 prior to the start of the
stent coating process (FIG. 2). The mandrel 15 controls the stent
position during drying and spraying. The mandrel 15 generally maintains
axial alignment of the stent, and causes the stent to rotate at generally
the same rate as the mandrel 15, which has a proximal end that fits into
a chuck. The chuck delivers a torque to the mandrel 15. The slots 36 and
38 provide a sufficient clearance to allow the mandrel 15 to rotate. The
mated grooves 63a, 63b (FIGS. 7A-7B) also provide this clearance for
rotation. Some heating gas will escape through the slots 36 and 38.

[0060]FIG. 8 shows a perspective view of the base housing 20, with the
portions identified as previously described. As mentioned earlier, the
base housing 20 includes a threaded fitting (hidden from view) that
receives the fitting for the gas supply. The diffuser housing 30 and
spacers/screens 40, 42 are received in the base housing 20. The walls
forming the drying chamber 32 extend out from the opening 22 of the base
housing 20 (see FIG. 1B).

[0061] For the drying systems described in US20110059228 and US20110000427
there is preferably an oven step for removing residual solvent from the
stent or scaffold. In an additional aspect of disclosure, the oven step
may be skipped as tests show that the dryer 10 and process as shown and
described may remove solvent at a sufficient rate during the process of
FIG. 2 to obviate an oven step. This is desirable as it reduces
manufacturing time for the medical device.

[0062] Twelve as-coated samples were collected to assess efficiency of the
dryer 10 with and without a later oven step. Those samples were processed
using inter pass dry temperature at 50 C. Those samples were divided into
two groups--Group A and Group B. The six group A samples were kept in a
tightly sealed vial and in the refrigerator prior to residual solvent
testing, and while the six group B samples proceeded with an additional
oven dry at 50 C for 30 minutes immediately after the final coating step,
then kept in the vial.

[0063] The residual acetone data for the two groups are listed in the
TABLE 1. The data shows that there is not much different between the
average of the residual acetone level between the two groups (between 1
to 2 micrograms). This is because the actual amount of a residual solvent
present in a coated stent can vary within a few micro-grams of a measured
amount, which is what TABLE 1 shows. Moreover, in some applications up to
5 μg of residual solvent remaining in the coating is considered
acceptable. Accordingly, the test suggests there may be no need to have
an oven bake step when using a dryer constructed in accordance with dryer
10.

[0064] A gas flow rate through the heater assembly 2 in FIG. 1 may be
monitored/controlled by a commercially available mass flow regulator (not
shown). For example, such a mass flow regulator may be used to operate an
adjustable valve coupling the gas supply line 2b to a gas source to
produce the desired flow rate. One example of a suitable mass flow
regulator is the Aalborg GFCS series programmable mass flow regulator. A
use of a mass flow regulator and related control system suitable for use
with aspects of the disclosure is described in U.S. application Ser. No.
12/540,302.

[0065] During a coating process, the dryer is not in use when the stent is
being coated. If the dryer is shut down or the flow rate reduced the
temperature of the gas at the entrance to the plenum 10 of the dryer 1
will decrease. If the stent is moved into position above the nozzle mouth
for drying and the valve opened to increase the flow rate, there will be
a period of transient flow. It is desirable to avoid a period of solvent
removal by transient gas flow, since the rate or amount of solvent
removal by transient flow can be difficult to predict. It is preferred,
therefore, that the stent is dried only during steady state flow
conditions.

[0066] If gas flow at the dryer is instead maintained at a constant rate,
then the temperature may be maintained. However, this wastes gas
resources. It would be desirable if the gas flow rate could be reduced
when the dryer is not in use while holding the gas temperature at a
constant value.

[0067] To meet this need, a closed loop control is preferably implemented
with a stent dryer system according to the disclosure, so that the gas
temperature may be maintained at variable flow rates. Referring to FIG.
9, a schematic of this closed-loop control is illustrated. A controller
300 continuously receives input temperatures at the entrance of the
plenum from a thermocouple 302 and the gas flow rate upstream of the
plenum entrance from a flow sensor 304. The controller 300 may be
programmed to reduce the gas flow rate when the dryer is not in use, and
increase the gas flow rate when the stent is ready to be moved into
position above the dryer mouth.

[0068] As the flow rate is adjusted by opening/closing the adjustable
valve 308, the controller senses a change in temperature from input
received at the thermocouple 302, at which point it will
increase/decrease the power delivered to the heating coils by affecting
control 306 for power so that the temperature remains constant,
regardless of the actual flow rate. Thus, according to this aspect of the
disclosure, a dryer system may be operated at variable flow rates during
a coating process while maintaining a substantially steady state gas flow
during the drying stage, or a minimal period of transient flow conditions
until a steady state condition is reached. This improves/maintains the
predictability of solvent removal during drying, minimizes down time and
allows gas resources to be conserved. The coated stent is almost
immediately subject to the drying step and dried in a manner that allows
the improved prediction of solvent removal. As discussed earlier, this is
a critical step in the process of producing a predictable release rate
for a drug-eluting stent and accurate assessment of whether the desired
drug-polymer coating weight has been reached.

[0069] After, or just prior to completion of an application of coating
composition on the stent, the controller 300 increases the gas flow
temperature to the drying gas flow rate. While the gas flow is being
increased, the controller 300 monitors the temperature at the plenum
entrance 2c by input received from the thermocouple 302 and the power
increased to the heating coils as necessary to maintain the temperature
of the exiting gas flow. Once the gas flow has reached the operating flow
rate and temperature, the stent is moved into position above the drying
chamber 32 and the housing 30 raised. The stent is rotated. After drying
is complete, the gas flow is again returned to the idle state and the
power to the heating coils decreased as necessary to maintain the same
gas flow temperature (based on input received from the thermocouple 302)
at/near location 2c. The process repeats until the desired coating weight
is obtained.

[0070] The above description of illustrated embodiments of the invention,
including what is described in the Abstract, is not intended to be
exhaustive or to limit the invention to the precise forms disclosed.
While specific embodiments of, and examples for, the invention are
described herein for illustrative purposes, various modifications are
possible within the scope of the invention, as those skilled in the
relevant art will recognize.

[0071] These modifications can be made to the invention in light of the
above detailed description. The terms used in claims should not be
construed to limit the invention to the specific embodiments disclosed in
the specification. Rather, the scope of the invention is to be determined
entirely by claims, which are to be construed in accordance with
established doctrines of claim interpretation.